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The hydrolysis and condensation of metal alkoxides M(OR)z allows the formation of oxopolymers or oxide colloids under mild conditions in solution. The molecular design of these precursors provides a chemical control over the formation of condensed phases. This can be conveniently performed via the chemical modification of alkoxides by nucleophilic species such as carboxylates or β-diketones. Condensation can be tailored with two chemical parameters; the hydrolysis ratio which leads to more condensed species and the amount of complexation which prevents condensation. Molecular clusters or colloidal particles can be obtained instead of precipitates. Moreover, non hydrolyzable complexing organic ligands lead to the formation of hybrid materials in which organic and inorganic moities are chemically bonded. These hybrid clusters and colloids are important starting points for producing solids and films with novel physical properties.

In this manuscript we describe several approaches to the preparation of molecule-sized fragments of inorganic solids. The first is the arrested precipitation of metal selenides in reverse micelles; the second is the arrested pyrolysis of molecular precursors to II–VI compounds; and the third is the chemical moderation of molecules-to-solids reactions.

Ultrasmall semiconductor particles (CdS, ZnS and PbS) are produced either by direct precipitation, γ-radiolysis or in mimetic membranes. A pure cluster powder is then prepared from a chemical capping reaction. The capping of the cluster surface by thiolate complexes permits the separation of aggregates without fusion. These capped clusters can be dispersed in different solvents such as sol-gel precursors allowing to prepare dense and optically transparent xerogels with semiconductor clusters grafted on oxide polymers. The nanocrystallites are characterized by X-ray diffraction, small-angle X-ray scattering and optical spectroscopy.

The molecular polychalcogenide complexes [M(Se4)2]2−, (M=Mn, Zn, Cd, Hg), [Sn(Se4)3]2−, and [Cu4Se12]2− can be converted to the corresponding binary semiconducting solids. DMF and DMSO solutions of these complexes react with Se-abstracting reagents such as CN− and (n-Bu)3P to yield the corresponding binary solids at 155 °C or less. Appropriate stoichiometric mixtures of [Cu4Se12]2− and [In3Se15]3− react to give CulnSe2. Appropriate stoichiometric mixtures of [Cd(Se4)2]2− and [Mn(Se4)2]2− give solid solutions Cd1−xMnxSe where O1−xMnxSe solid solutions were characterized by variable temperature magnetic measurements. Depending on the reaction conditions, metal precursor complex and Se-abstracting reagent used, the semiconductor particle size ranges from the quantum-size to the bulk regime. This method of synthesis produces the little known γ-MnSe in pure form.

Powders of pure silicon and gallium arsenide were reacted with molten silicate glass of a composition used in our prior work on CdS and CdTe quantum dots in glass [1–4]. After the Si or GaAs agglomerated particles had reacted with the melt for an hour, the material was solidified to obtain a gray-black glass. Ground powders of this glass were again mixed with additional base glass powders to dilute the concentration of the semiconductor, remelted, and cast into glass. Samples of silicon-in-glass showing green photoluminescence peaked at 530 nm were obtained. The GaAs-in-glass samples show absorption features in the optical spectra which suggest the possibility of quantum confinement.

An Organically-modified silicate (Ormosil) which contains 28 wt% polydimethylsiloxane and 72 wt% silica has been used as a matrix to fabricate CdS-doped glassy nanocomposites by using the sol-gel method. Samples were prepared both in bulk and film forms. High semiconductor concentrations, up to 20 wt% CdS could be obtained and the semiconductor microcrystallites were formed at temperatures much lower than that of the purely inorganic glass matrix. Preparation methods of these CdS-doped Ormosils are described. Optical behaviors of these nanocomposites under normal and intense photon influx were studied.

In this paper we report on the mechanism of the solid state conversion of a series of IINI precursors of general formula (R4N+)4[S4M4(SPh)16]4− (R=Me, Et; M=Cd, Zn) to the bulk metal sulfide structure. This family of cluster compounds was first reported by Dance et al [1]. On heating these solids in vacuum or inert atmosphere, we find that they convert to the corresponding extended metal sulfide lattice in two discrete reaction steps. The overall reaction appears to be general, but there are interesting differences in the rates and mechanistic details as R and M are varied. The first step, ˜200°C, can be characterized as a nucleophilic substitution or elimination initiated by the attack of a fragment of the anion cluster on the tetra-alkyl ammonium counterion. The intermediate solid produced in this step has been isolated and recrystallized. It consists of charge neutral M10S16Ph12 clusters that retain the cluster size present in the starting material but appear to aggregate to varying degrees in the solid or solution.

Films and monoliths, containing clusters (sizes < 5 nm) of the binary semiconductor CdS and sandwiched CdS-PbS, were prepared via multifunctional inorganic-organic sol-gel processing. As a sulfur source, hexamethyldisilylthiane was employed. In precursor sols, the metal sulfide clusters are carrying functionalized silanes acting as stabilizing centers as well as inorganic and organic network formers. Hydrolysis and condensation produces an inorganic network yielding viscous liquids useful to prepare optically transparent films or monoliths. The final organic cross-linking at T < 100°C results in materials of variable spectral response, thickness and optical density. In preliminar degenerate four-wave mixing experiments, self-diffraction from laser-induced gratings was observed on unsupported 200 μm thick CdS-PbS doped monoliths. The maximum first order grating efficiency, measured at different wavelengths between 490 and 520 nm, was 0.5 · 10−3 and the corresponding calculated effective third order susceptibility was of the order of 10−9 esu.

The decomposition of colloidal spheres of mixed rare earth hydroxycarbonate to the oxide has been studied by three methods. a) The spheres are treated in a thermal analysis apparatus up to 1100°C and afterward successive reruns were made and specimens cooled rapidly at temperatures where intermediate or final products are formed. X-ray diffraction patterns and HREM studies were made of each sample thus produced. b) The spheres were heated to 450°C in the specimen chamber of a mass spectrometer and the gaseous decomposition products were monitored continuously. c) The colloidal spheres were introduced into the electron microscope and the decomposition followed at high resolution as evolution to the oxide was induced by the electron beam. The results are compared and contrasted.

A comparison of the activities of conventionally precipitated metals on charcoal, deposited organometallic clusters and metal colloids for liquid-phase hydrogenation has been made under standard conditions in order to assess the effect of particle size. The strong SMSI-effect known for TiO2-supports was simulated on charcoal by doping the surface with small amounts of Ti(0) and subsequent oxygenation.

Metal particles in aqueous solution can be surface-modified by chemisorbed molecules and/or by a second deposited metal. The electron density in the metal particles is varied by controlled electron transfer from free radicals.’ These modifications lead to significant electronic changes in the metal particles, the result being changes in the optical properties and in chemical reactivity.

The physicochemical properties also change during the transition from the atomic to the metallic state with increasing particle size. In the case of silver, oligomeric non-metallic clusters can be stabilized in solution for days, time enough to study many of their properties. In other cases (Au, Cu, Pb), the clusters are short-lived; they are investigated by fast kinetic methods such as pulse radiolysis.

Ligand-stabilized bimetallic Au/Pd colloids may be prepared from 180–200Å Au colloids which can be covered by Pd shells of varying thickness. They can be isolated as solid materials without agglomeration. HREM observations indicate a narrow particle size distribution and reveal a well-defined Au(core)/Pd(shell) structure. This is convincingly confirmed by EXAFS data. Thermal treatment leads to Au/Pd alloy formation at ≥ 500 K, as evidenced by the appearance of characteristic Au-Pd distances in the EXAFS.

Reduction of triphenylphosphine-containing Pt(II) complexes with H2, HCOOH and Na-amalgam was studied to compare the results with those for similar reductions of analogous Pd(II) complexes. Clusters of compositions [Pt(PPh2)]n, n=6–10, have been found to be the products of the reduction reactions under consideration.

A new reduction method for the preparation of the molybdenum halides MoCl3(THF)3 and MoCl4(THF)2 in high yield and with high purity directly from MoCl5 is described. The preparation of pure starting materials is crucial to the success of the subsequent chemical reduction. Reduction of MoCl3(THF)3, MoCl4(THF)2 or WCl4 in THF with LiBEt3H at room temperature did not.result in formation of Mo and W as anticipated but instead resulted in formation of nanophase M2CM = Mo and W binary metal carbides. These species were characterized by SEM, TEM, energy dispersive spectroscopy, electron diffraction, elemental analysis, thermogravimetric analysis and X-ray diffraction techniques. These techniques showed the black solids were crystalline and comprised 1–2 nm sized crystallites Which could be grown by heating to higher temperatures (450 – 500°C). The solids isolated from these experiments could be redispersed in THF to form colloidal black solutions.

Nanometer-sized metal particles have been prepared by chemical and photochemical reduction of homogeneous solutions of metal-organic rhodium(I) compounds. Reduction of [(COD)RhCl]2 with LiBEt3H in THF resulted in the formation of solutions containing crystalline, mono-dispersed Rh particles with average sizes of 2–4nm as determined by TEM and SAXS. The species [(COD)RhH]4 was isolated during the formation of the Rh colloids but was found not to be an intermediate in the reduction of Rh(I) to Rh(0). The catalytic activity of these highly dispersed metals towards pyrene hydrogenation has been investigated. Photochemical reduction of [(ethylene)2Rh(OEt)]2 at room temperature also resulted in formation of Rh as shown by TEM, EDS, ED and X-ray diffraction.

The reaction of [Fe(CO)4]2− with Ag+ in tetrahydrofuran or acetonitrile solution affords a series of silver clusters stabilized by Fe(CO)4 pseudo-ligands, formally arising from oligomerization of [AgFe(CO)4]− and their subsequent condensation with Ag+/O/−. The [Ag4Fe4(CO)16]4−, [Ag5Fe4(CO)16]3−, and [Ag13Fe8(CO)32]4− anions have been structurally characterized by X ray studies, while the formulation of all the remaining species is presently based on analytical and spectroscopic characterization, as well as their chemical behavior. Attempts to isolate the [Ag6Fe4(CO)16]2− salts in a crystalline state were unsuccessful due to their ready oligomerization to completely insoluble material of [(Ag+)n{[Ag6Fe4(CO)16]2−}n+1](n+2)− composition. This material is depolymerized by dimethylsulfoxide or by addition of stoichiometric amounts of amines and, upon dissolution, gives the [Ag6Fe4(CO)16]2− dianion.

Voltammetric studies of Au55(PPh3)12Cl6 (kindly provided by Prof. G. Schmid, Essen, Germany) in solutions of CH2Cl2 and THF with Pt electrodes show that the reduction of the cluster is a multistep, irreversible process. Precipitation of colloidal metal or any high molecular super-cluster is observed when stabilising. NBu+ –ions are absent and when the reduced solution is reoxidised.

The parent cluster and the precipitate were studied by X-ray powder diffraction and UV-vis spectroscopy. The origin of the diffuse lines and the low-angle diffractions will be discussed. They suggest that the Au55 particles flocculate by changing their electric charge in the electrochemical redox reactions. The floc is unstable and congulates to particles of 25 Å and larger.

A series of metal-cluster-organically modified silicate, or Ormosil, nanocomposites were prepared by the sol-gel method. Each of the metals (Ag, Au, Cu, and Pt), was incorporated individually into the Ormosil matrices at various weight percents by reduction of the appropriate metal salt. The preparation for these metal-cluster doped-Ormosils is described. Ormosil proved to be an ideal matrix material, offering excellent transparency and good cracking resistance. The presence of the metal clusters inside the Ormosil matrices was detected by transmission electron microscope and verified by X-ray diffraction. Optical absorption. measurements of these nanocomposites are also presented.

A series of palladium clusters containing from 4 to some hundred Pd atoms in the metal skeleton was prepared and characterized with structural data as well as by chemical properties including catalytic activity. An agreement between X-ray and EXAFS data was obtained for single-crystal low-molecular clusters. Giant clusters approximated as Pd56L60(OCOCH3)180 (L=Phen, Bipy) and Pd561Phen60O60X60 (X = PF6, CIO4, SO4, BF4, CF3COO)were characterized with TEM, SAXS, EXAFS, NMR, and magnetic susceptibility data. The clusters consisting of close-packed metal core and L, X ligands at its periphery could be considered as a bridge between low-molecular clusters and the particles of colloidal metals.

In the widest sense, a cluster is an aggregate of molecules or atoms. Many clusters contain metal atom groups Mn in which the metals are chemically bonded to each other. To decide whether metal-metal bonds are present, the comparison with bonding in pure metals is often made.